The use of fiber optic cables for the transmission of information was introduced years ago. Recently, with the growing demand for transmission of large amounts of information at high speed, the usefulness of such transmission of optical signals becomes evident. (G P Agrawal “Fiber Optic Communication Systems” John Wiley & Sons, 2002). The transmission of this information typically is done by means of binary digits (logical ones and zeros). But also analog signals are allowed, such as cable television signals.
In the 90's, Wavelength Division Multiplexing (WDM) was introduced at a commercial level. This is a technique that made possible to transmit multiple wavelengths in parallel; and, thus, increase the optical fiber capacity of carrying information.
Moreover, since some years ago, the research community is investigating how should be the deployment of fiber optic networks, in order to reach the final user home (Fiber To The Home, FTTH). It is intended that, in such networks, a single distribution fiber can carry one wavelength per user. Then, the maximum output is obtained when one reaches the maximum number of wavelengths possible in a single fiber. Therefore, what is pretended is to make as narrow as possible the separation between channels.
A fiber optic communications system, in its most basic scheme, consists of an emission block, called optical transmitter, which is intended to transform the information into light (usually that incoming information is driven in the form of electrical signal); a transmission media, which is optical fiber; and a reception block, which is intended to transform the optical information received into an electrical signal containing such information. This last block is called optical receiver. It should be noted, that the transmitter block usually contains an optical light source, which can be, for example, a laser diode or a light emitting diode (LED); while the optical receiver contains an optical detector, which can be, for example, a photo-diode (PIN or avalanche) or a photo-transistor. The optical transmitter and receiver include connectors capable to couple and connect them to the optical fiber.
Regarding the transmitter, it is composed of a light source, preferably a laser, and modules (optional, depending on the modulation format), placed next to it, in order to introduce the information to be transmitted. The modulation formats can be different, and most of them are simple adjustments to their versions used in the field of radio and satellite communications. Thus, we have modulations of phase, amplitude, frequency, etc. for transmitting the information.
It is known, in the field of optical receivers, the use of direct detection optical receivers and the use of coherent optical receivers.
The architecture of direct detection optical receivers is based, primarily, on a photo-detector followed by an amplifier, and signal processing circuits. Thus, the receiver converts an optical signal into and electrical signal proportional to the incident optical power, which is subsequently processed. The main drawbacks of optical direct detection are the selection of optical channel and the noise, meaning noise any unwanted change of the signal that carries information on the communications system. It is also noteworthy that these receivers are not able to recover the phase of the optical signal. Thus, the possibility of using optical phase modulation and similar is automatically excluded.
On the other hand, coherent detection optical receivers, that are described in (Silvelo Betti, Giancarlo of Marchis, Eugenio Iannone, “Coherent optical communications systems,” John Wiley & sons, inc. 1995), receive an optical signal carrying information and couple it with the light coming from a local laser oscillator, for obtaining the information in baseband or intermediate frequency at the electric output of the photo-detector, due to the interference between the two light beams, in a manner similar to the receivers of current radio systems. When the information is down-converted to a baseband signal, the receiver is said to be homodyne. In other cases of reception, it is said to be heterodyne.
Importantly, homodyne detection improves the performances of an optical receiver, like sensitivity and frequency selectivity of the optical transmission, but has the disadvantage of needing lasers highly coherent and an optical phase-locked loop (oPLL). This oPLL is a device that still is at experimental stage, having a complex design and a high cost. That is the reason why, at present moment, homodyne detection is possible only performed in laboratory experiments, like the types of oPLL that have been demonstrated: mainly, the balanced loop, and the Costas Loop (see L G Kazovsky Balanced phase-locked loops for optical homodyne receiver: Performance analysis, design considerations, and laser linewidth requirements “Journal of Lightwave Technology, Volume 4, Issue 2, February 1986 Page (s): 182-195, or S. Norimatsu and K. Iwashita “PLL propagation delay-time influence on linewidth requirements of optical PSK homodyne detection” Journal of Lightwave Technology, Volume 9, Issue 10, October 1991 Page (s): 1367-1375). All of them require a laser spectral linewidth less than 0.1% of the transmission speed, and a loop delay lower than their coherence time.
There is now considerable research activity to solve these problems, because this type of reception is theoretically described as the most advantageous (see for example “Homodyne Phase-Shift-Keying Systems: Past and Future Challenges Opportunities” L. Kazovsky in the Optical Fiber Communications Conference -OFC'2005-, records OTuL3, Anaheim, California, USA, March 2005). Recently, some vanguard homodyne receivers have been proposed, such as S. Camatel (Electronics Letters, Volume 40, Number 6, June 2004), where the oPLL operates aided by an electrical sub-carrier, which can lock the phase of the received signal at high speed, but requires a coherent and high-cost laser. Other proposals, such as those of R. Noe (U.S. patent 2004/0091066 “Apparatus and Method for a Carrier Recovery” of 13/05/2004) or M.G. Taylor (U.S. patent 2004/0114939 “Coherent Optical Detection and Signal Processing Method and System of 17/06/2004) are based on estimate and recover the carrier phase using a digital signal processor. Such estimation today is unlikely to be implemented for high speed transmission over optical fiber.
Given the nowadays expectations of growth of fiber optic networks, and the benefits of homodyne reception, we understand that there is a need for such a reception system, with all its benefits in terms of sensitivity and selectivity; that can be implemented easily with very simple components, and can be of potential low cost.